| dc.description.abstract | Rapid Distortion Theory is used to perform calculations of unsteadily-forced initially
isotropic turbulence so that the physics of such flows can be better understood. The
results of these calculations show that there are three distinct regimes of physical behavior
for the kind of turbulence that we are considering: (1) turbulence that is forced
at a relatively low frequency in which the kinetic energy settles down to a constant
value at later times, (2) turbulence that is forced at a slightly higher frequency in
which the kinetic energy value oscillates for a time, but then increases dramatically,
and (3) turbulence that is forced at a relatively high frequency in which the kinetic
energy evolution exhibits a periodic behavior. To better understand the role of the
rapid pressure-strain correlation, these results are also compared to Inertial Model
results for the same set of forcing frequencies. The results of this comparison show
that the rapid pressure plays a key role in determining the stability characteristics of
unsteadily-forced turbulence. The evolution equation for kinetic energy is then used
to propose a model that describes the behavior approximately in terms of a time lag
between applied mean strain and the Reynolds stress. This model suggests that the
different responses under the different frequencies of forcing correspond to different
stress-strain time lags. Overall, then the results indicate that rapid pressure serves to
create a time lag between applied stress and strain, and it is the extent of this time
lag that causes turbulence to respond differently under various frequencies of forcing. | en |
